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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Sat Mar 07, 2009 1:43 pm 
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darus67 wrote:
But if you just put a single diode in series with the load, you'll only get current flowing on half of each cycle.
This will effectively cut your power in half.


Ah, good point. I wasn't thinking about power control. That makes a lot more sense.

I'm going to do the table-top model today (maybe). I bought some more wire (in the form of a heating element with a stupid long coil) from the Ax today- we'll see how this turns out.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 2:11 pm 
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OK, I'm getting frustrated. I got my nichrome in today, but I can't get this thing to get hot enough to do anything before the fuse blows.

Not getting it.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 3:26 pm 
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I loaded it up with the some 32 guage wire and it worked fine, so....resistance issues. With the 20 gauge loaded the resistance was just under 2 ohms (with no fluctuations this time compared to the 22 gauge galvanized which fluctuated wildly), the 32 gauge however was around 18 ohms, so big difference.

The 32 gauge deflects so easily though, I just can't see doing anything large with it in any kind of accurate fashion.

Any ideas how I can make the 20 gauge wire work?

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 3:35 pm 
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is your transformer ac/dc or ac/ac? aren't most of those dc devices?


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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 3:52 pm 
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It's ac/ac. I may have to add that rectifier to see if it helps.

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 4:18 pm 
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Perhaps your transformer just isn't big enough?

In certain instances, size matters.


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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 7:38 pm 
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Let's do math again.

Your transformer is a 24V transformer, right? At 24V and 2 ohms resistance, you'll push 12A through that wire. P=VI=I^2*R; that's 288W(!), which is more (by a long shot) than your average soldering iron uses, AND 12 times the rated current of that transformer. In fact, I daresay it's a good thing that your fuse went, or you'd have had something really, REALLY bad happen (fire? Melted wire? Who can say what the weakest link would have been?).

Even if you put the rectifier in, you're only killing half the power, and very, very few rectifiers will work at 12A peak current. You'd need heatsinking at the very least.

So what DO you need to do? My 10W wire cut nicely- let's do some math based on 50% higher. P = I^2*R = V^2/R so for P = 15, I ~= 2.75A and V ~= 5.5V. I might consider using (as Theo suggested before) an ATX power supply from a PC. One or another of the rails will need a load to pull the supply into regulation- it might be the 5V rail, in which case your wire provides the load quite nicely. If it's another one that's needed, well, Ax-man has a few high power resistors available, and I'm willing to be that if it's not the 5V supply that needs the load, it'd be the 3.3V supply.

A 6V supply might work okay- perhaps a battery charger for 6V hobby battery packs? Or 7.2V packs? That might work nicely- "quick chargers" for RC planes tend to charge at fairly high amperages, with little or no check on their current. In this case, you'd want to buy the cheapest one possible, as higher quality units would attempt to limit their charge current to preserve the life of the battery.

Here's a thought as to why the dimmer switch isn't a great solution- you want to adjust this voltage linearly, but that dimmer switch is set up to dim a logarithmic light source. Usually it's a cheat- they do two linear regions which nicely approximate a logarithmic scale. The reason for that is that light, like sound, is perceived to increase in a linear fashion when its intensity increases logarithmically. For this, though, you want a linear increase- the logarithmic increase would give you little to no increase in temperature for a while, then the temperature would suddenly skyrocket.

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Last edited by Theo on Mon Mar 09, 2009 8:00 pm, edited 1 time in total.
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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 8:37 pm 
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OK, couple things. It's hard for me to discuss stuff in batch mode like this.

Obviously I don't turn the think up to 11 and then turn it on, so melting down shouldn't be a concern. I of course ran it without a fuse and with low resistance wire when I started, the only price of that was a burned out transformer and dimmer.

I am thinking that the dimmer isn't ramping up power smoothly and the first step is beyond the current of the fuse which is why I'm getting no heat and a burned-out fuse right away.

Unfortunately everybody's solutions involve dumping what I have and doing something completely different instead of exploring the options in the existing domain. I have a laundry list of solutions that potentially make more sense and would be less trouble, but that wouldn't teach me much. At some point I may have to bail and go that direction because I need to make some progress on this cone for UMERG.

All I really need to do is get some heat into my 20 gauge wire without the fuse blowing, looking at max power isn't necessarily meaningful for this goal.

So will the rectifier accomplish that, or is there some way to divide the power coming out of the dimmer into the transformer?

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Mon Mar 09, 2009 9:46 pm 
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Adding a second transformer to step down the potential volts again?

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 Post subject: Re: Hot Wire Post Mortem
PostPosted: Tue Mar 10, 2009 7:02 am 
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noise wrote:
Unfortunately everybody's solutions involve dumping what I have and doing something completely different instead of exploring the options in the existing domain. I have a laundry list of solutions that potentially make more sense and would be less trouble, but that wouldn't teach me much. At some point I may have to bail and go that direction because I need to make some progress on this cone for UMERG.

All I really need to do is get some heat into my 20 gauge wire without the fuse blowing, looking at max power isn't necessarily meaningful for this goal.

So will the rectifier accomplish that, or is there some way to divide the power coming out of the dimmer into the transformer?


You keep talking about learning- maybe the lesson here is when to abandon a solution that will never work and move on to one that will?

What is the amperage of your fuse? Have you calculated the delivered power to your wire at that current? If it's a 1A fuse, your maximum delivered power will be 2W, which isn't going to cut butter.

You HAVE to look at max power. You have a center-tap 120V-24V transformer, which means you're going to get either 12V or 24V out of the thing. You have a two-ohm load which means that you're either going to be pushing 6A or 12A through the load- which implies either 288 or 144W of delivered power. If you put the rectifier in there, you can reduce the power by 1/2, but you're going to need to find a rectifier that can handle a minimum of 6A of instantaneous current, which is close to half a watt of instantaneous power (6A x .7V). Your average power will be much lower, but 6A is still a lot of current.

I guess there's one final suggestion I can make, if you're bound and determined to stick with your current setup. Disconnect your load, remove your fuse, put the dimmer on the load side of the transformer, adjust the dimmer until the output reads 6V or so, then take a piece of duct tape and fix the knob in that position. Disconnect the power, remove the fuse (or put in a bigger fuse), and reconnect your load. It might work, it might not- I'm not sure how the dimmer will react to being powered by lower-than-line voltage, and your meter is probably not a true-RMS meter, so it's going to be ball-parking your RMS voltage based on the assumption that you're measuring a sine wave when in fact you're going to be measuring a chopped sine wave.

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